Resident Permits supersede any visa you enter china with. However the type of visa you entered is still important as to what you are legally allowed to do in China. For example if you enter on a student visa and get a resident permit you can not work legally. You would need to leave the country and apply for a work visa. Upon re-entry you would need to get a new resident permit.

It is important in China where and by whom your resident permit was applied. Whoever applied for your res permit has the right to cancel it also. This is why most employment contracts have the clause that you cannot work for any other business.

A business or company needs a special permit from the government that will allow them to employ foreigners.

Once you have your resident permit it becomes your multi entry visa. You can leave and return as much as you like on a valid resident permit.

Not quite. You can transition form student to work and back without leaving the country and without getting a new visa. You do, however, need to get a new residence permit. On the residence permit there is a field for "Purpose of Residence" which would need updated. The original entry visa becomes irrelevant once you have a residence permit.
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Rincewind42Oct 21 '13 at 6:19